Wireless LANs and IEEE MAC

Wireless LANs and IEEE 802.11 MAC
24 February 2019

Overview

Applications of Wireless LANs
Key application areas: LAN extension cross-building interconnect nomadic access ad hoc networking A wireless LAN makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used. The reasons for this included high prices, low data rates, occupational safety concerns, and licensing requirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly. [PAHL95] lists four application areas for wireless LANs: LAN extension, cross-building interconnect, nomadic access, and ad hoc networks.

Infrastructure Wireless LAN
Stallings DCC9e Figure 17.3 suggests the differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. In the former case, the wireless LAN forms a stationary infrastructure consisting of one or more cells with a control module for each cell. Within a cell, there may be a number of stationary end systems. Nomadic stations can move from one cell to another. In contrast, there is no infrastructure for an ad hoc network. Rather, a peer collection of stations within range of each other may dynamically configure themselves into a temporary network.

Ad Hoc Networking temporary peer-to-peer network (no infrastructure)
An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting. Stallings DCC8e Figure 17.3 suggests the differences between a wireless LAN that supports LAN extension and nomadic access requirements and an ad hoc wireless LAN. In contrast to the previous slide, there is no infrastructure for an ad hoc network. Rather, a peer collection of stations within range of each other may dynamically configure themselves into a temporary network.

Wireless LAN Requirements
THROUGHPUT – should make efficient use of medium NUMBER OF NODES- hundreds of nodes across multiple cells CONNECTION TO BACKBONE LAN – use of control modules SERVICE AREA – coverage area of 100 to 300m BATTERY POWER CONSUMPTION – reduce power consumption while not in use TRANSMISSION ROBUST AND SECURITY– reliability and privacy/security COLLOCATED NETWORK OPERATION – possible interference between LANs LICENSE-FREE OPERATION – not having to secure a license for the frequency band used by the LAN HANDOFF/ROAMING– enable stations to move from one cell to another DYNAMIC CONFIGURATION- addition, deletion, relocation of end systems without disruption A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs: Throughput: The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity. Number of nodes: Wireless LANs may need to support hundreds of nodes across multiple cells. Connection to backbone LAN: In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks. Service area: A typical coverage area for a wireless LAN has a diameter of 100 to 300 m. Battery power consumption: Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to monitor access points constantly or engage in frequent handshakes with a base station is inappropriate. Typical wireless LAN implementations have features to reduce power consumption while not using the network, such as a sleep mode. Transmission robustness and security: Unless properly designed, a wireless LAN may be especially vulnerable to interference and eavesdropping. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping. Collocated network operation: As wireless LANs become more popular, it is quite likely for two or more wireless LANs to operate in the same area or in some area where interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN. License-free operation: Users would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN. Handoff/roaming: The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another. Dynamic configuration: The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.

Wireless LAN Technologies
spread spectrum LANs mostly operate in ISM (industrial, scientific, and medical) bands no Federal Communications Commission (FCC) licensing is required in USA OFDM LANs orthogonal frequency division multiplexing superior to spread spectrum operate in 2.4 GHz or 5 GHz band infrared (IR) LANs individual cell of IR LAN limited to single room IR light does not penetrate opaque walls Wireless LANs are generally categorized according to the transmission technique that is used. All current wireless LAN products fall into one of the following categories: Spread spectrum LANs: This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (industrial, scientific, and medical) 2.4-GHz microwave bands so that no Federal Communications Commission (FCC) licensing is required for their use in the United States. OFDM LANs: For higher speeds, a technology known as orthogonal frequency division multiplexing is superior to spread spectrum, and products with this technology are now common. These LANs typically operate in either the 2.4-GHz band or the 5-GHz band. Infrared (IR) LANs: An individual cell of an IR LAN is limited to a single room, because infrared light does not penetrate opaque walls. This type of LAN is little used The remainder of this section provides a brief overview of some aspects of spread spectrum and OFDM LANs.

IEEE Standards In 1990, the IEEE 802 Committee formed a new working group, IEEE , specifically devoted to wireless LANs, with a charter to develop a MAC protocol and physical medium specification. Since that time, the demand for WLANs, at different frequencies and data rates, has exploded. Keeping pace with this demand, the IEEE working group has issued an ever-expanding list of standards (Stallings DCC9e Table 17.1). Stallings DCC9e Table 17.2 briefly defines key terms used in the IEEE standard. The Wi-Fi Alliance The first standard to gain broad industry acceptance was b. Although b products are all based on the same standard, there is always a concern whether products from different vendors will successfully interoperate. To meet this concern, the Wireless Ethernet Compatibility Alliance (WECA), an industry consortium, was formed in This organization, subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance, created a test suite to certify interoperability for b products. The term used for certified b products is Wi-Fi. Wi-Fi certification has been extended to g products,. The Wi-Fi Alliance has also developed a certification process for a products, called Wi-Fi5. The Wi-Fi Alliance is concerned with a range of market areas for WLANs, including enterprise, home, and hot spots.

IEEE Terminology Stallings DCC9e Table 17.2 briefly defines key terms used in the IEEE standard.

IEEE Architecture Stallings DCC9e Figure 17.4 illustrates the model developed by the working group. The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared wireless medium. A BSS may be isolated or it may connect to a backbone distribution system (DS) through an access point (AP). A simple configuration is shown in Figure 17.4, in which each station belongs to a single BSS; that is, each station is within wireless range only of other stations within the same BSS. It is also possible for two BSSs to overlap geographically, so that a single station could participate in more than one BSS. Further, the association between a station and a BSS is dynamic. Stations may turn off, come within range, and go out of range.

IEEE 802.11 - BSS basic service set (BSS) building block
may be isolated may connect to backbone distribution system (DS) through access point (AP) BSS generally corresponds to cell DS can be switch, wired network, or wireless network Ad-hoc network: independent BSS (IBSS) with no AP The AP functions as a bridge and a relay point. In a BSS, client stations do not communicate directly with one another. Rather, if one station in the BSS wants to communicate with another station in the same BSS, the MAC frame is first sent from the originating station to the AP, and then from the AP to the destination station. Similarly, a MAC frame from a station in the BSS to a remote station is sent from the local station to the AP and then relayed by the AP over the DS on its way to the destination station. The BSS generally corresponds to what is referred to as a cell in the literature. The DS can be a switch, a wired network, or a wireless network. When all the stations in the BSS are mobile stations, with no connection to other BSSs, the BSS is called an independent BSS (IBSS). An IBSS is typically an ad hoc network. In an IBSS, the stations all communicate directly, and no AP is involved.

Extended Service Set (ESS)
possible configurations: simplest is each station belongs to single BSS can have two BSSs overlap a station can participate in more than one BSS association between station and BSS dynamic ESS is two or more BSS interconnected by DS appears as single logical LAN to LLC An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN but can be any communications network. The extended service set appears as a single logical LAN to the logical link control (LLC) level. Stallings DCC9e Figure 17.4 indicates that an AP is implemented as part of a station; the AP is the logic within a station that provides access to the DS by providing DS services in addition to acting as a station. To integrate the IEEE architecture with a traditional wired LAN, a portal is used. The portal logic is implemented in a device, such as a bridge or router, that is part of the wired LAN and that is attached to the DS.

Medium Access Control

Medium Access Control MAC layer covers three functional areas:
reliable data delivery access control security The IEEE MAC layer covers three functional areas: reliable data delivery, access control, and security. This section covers the first two topics.

Medium Access Control The working group considered two types of proposals for a MAC algorithm: distributed access protocols, which, like Ethernet, distribute the decision to transmit over all the nodes using a carrier sense mechanism; and centralized access protocols, which involve regulation of transmission by a centralized decision maker. A distributed access protocol makes sense for an ad hoc network of peer workstations (typically an IBSS) and may also be attractive in other wireless LAN configurations that consist primarily of bursty traffic. A centralized access protocol is natural for configurations in which a number of wireless stations are interconnected with each other and some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time sensitive or high priority. The end result for is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access control mechanism with an optional centralized control built on top of that. Stallings DCC9e Figure 17.5 illustrates the architecture. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. PCF is built on top of DCF and exploits features of DCF to assure access for its users.

DCF and PCF PCF and DCF operate concurrently within the same BSS.
The two access methods alternate, with a contention-free period (CFP) followed by a contention period (CP). DCF: fundamental access method of IEEE MAC, implemented in all STAs. known as CSMA/CA

PCF Supports time-bounded services.
Lets stations to have priority access to the wireless medium. Polling stations one by one (centralized operation) Coordinated by Point Coordinator (PC), typically collocated with the AP. PCF has higher priority than the DCF. Beacon frame is a management frame that maintains the synchronization of the timers in the stations and delivers protocol related parameters.

Distributed Coordination Function (DCF)

DCF MAC Requirements To avoid interference among simultaneous transmissions But enable as many non-interfering transmission as possible Maintain fairness among transmissions No centralized coordinators: fully distributed operations No clock synchronization: asynchronous operations

CSMA/CA DCF sub-layer uses CSMA/CA
if station has frame to send it listens to medium if medium idle, station may transmit else waits until current transmission completes No collision detection since on wireless network, so use collision avoidance (backoff and RTS/CTS) DCF includes delays that act as a priority scheme DIFS: DCF inter-frame space SIFS: short inter-frame space (SIFS < DIFS) The DCF sublayer makes use of a simple CSMA (carrier sense multiple access) algorithm. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise the station must wait until the current transmission is complete before transmitting. The DCF does not include a collision detection function (i.e., CSMA/CD) because collision detection is not practical on a wireless network. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a single delay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail.

Backoff Interval When channel is busy, choose a backoff interval in the range [0, cw] (concept similar to non-persistent). Count down the backoff interval when medium becomes idle. Count down is suspended if medium becomes busy again. When backoff interval reaches 0, transmit RTS. Binary exponential backoff in DCF: When a node fails to receive CTS, cw is doubled up (up to an upper bound). When a data transfer completes successfully, cw is reset to cwmin. 21

IEEE 802.11 Medium Access Control Logic
Using an IFS, the rules for CSMA access are as follows (Stallings DCC8e Figure 17.6): 1. A station with a frame to transmit senses the medium. If the medium is idle, it waits to see if the medium remains idle for a time equal to IFS. If so, the station may transmit immediately. 2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over. 3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off a random amount of time and again senses the medium. If the medium is still idle, the station may transmit. During the backoff time, if the medium becomes busy, the backoff timer is halted and resumes when the medium becomes idle. 4.If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred. To ensure that backoff maintains stability, binary exponential backoff, described in Chapter 16, is used. Binary exponential backoff provides a means of handling a heavy load. Repeated failed attempts to transmit result in longer and longer backoff times, which helps to smooth out the load. Without such a backoff, the following situation could occur: Two or more stations attempt to transmit at the same time, causing a collision. These stations then immediately attempt to retransmit, causing a new collision.

Basic CSMA/CA operations
Fig (Leon-Garcia)

Transmission without RTS/CTS
Fig (Leon-Garcia)

Carrier Sensing Transmission range Sensing range
Sensing range  Transmission range Problems Hidden terminal problem Exposed terminal problem Note: contention matters only at the receiver’s terminal

Hidden Terminal Problem
X No carrier  OK to transmit

Exposed Terminal Problem
Y X Presence of carrier  holds off transmission

Solutions MACA [Karn 1990] MACAW [Bharghanvan 1994] IEEE 802.11
Proposes to solve the hidden terminal problem by RTS/CTS dialog MACAW [Bharghanvan 1994] Increasing reliability by RTS/CTS/DATA/ACK dialog IEEE Distributed Coordination Function (DCF) Also use RTS/CTS/DATA/ACK dialog

RTS/CTS dialog (1) Any node hearing this RTS will defer medium access

RTS/CTS dialog (2) Any node hearing this CTS will defer medium access

RTS/CTS/DATA/ACK dialog
Defer Defer Data ACK

Transmission with RTS/CTS
Fig (Leon-Garcia)

IEEE 802.11 DCF (1) CSMA/CA Contention-based random access
Collision detection not possible while transmitting Uses RTS/CTS exchange to avoid hidden terminal problem Any node overhearing a CTS cannot transmit for the duration of the transfer. Any node overhearing an RTS cannot transmit for the duration of the transfer (to avoid collision with ACK) Uses ACK to achieve reliability

IEEE 802.11 DCF (2) Carrier sense in 802.11 Physical carrier sense
Virtual carrier sense using Network Allocation Vector (NAV) RTS/CTS specify duration of subsequent DATA/ACK NAV is updated based on overheard RTS/CTS /DATA Collision avoidance Nodes stay silent when carrier sensed busy (physical/virtual) Backoff intervals are used to reduce collision probability

Backoff Interval When channel is busy, choose a backoff interval in the range [0, cw] (concept similar to non-persistent). Count down the backoff interval when medium becomes idle. Count down is suspended if medium becomes busy again. When backoff interval reaches 0, transmit RTS. Binary exponential backoff in DCF: When a node fails to receive CTS, cw is doubled up (up to an upper bound). When a data transfer completes successfully, cw is reset to cwmin.

IEEE CSMA/CA – Example DIFS: DCF inter-frame space SISF: short inter-frame space

Disadvantages of IEEE 802.11 DCF
High power consumption Hidden terminal problem not totally solved (e.g., collision of RTS) Exposed terminal problem not solved Fairness problem among different transmitting nodes Only providing best-effort service

Priority IFS Values SIFS (short IFS) PIFS (PCF IFS) DIFS (DCF IFS)
for all immediate response actions (see later) PIFS (PCF IFS) used by the centralized controller in PCF scheme when issuing polls DIFS (DCF IFS) used as minimum delay for asynchronous frames contending for access The preceding scheme is refined for DCF to provide priority-based access by the simple expedient of using three values for IFS: • SIFS (short IFS): The shortest IFS, used for all immediate response actions, as explained in the following discussion • PIFS (point coordination function IFS): A midlength IFS, used by the centralized controller in the PCF scheme when issuing polls • DIFS (distributed coordination function IFS): The longest IFS, used as a minimum delay for asynchronous frames contending for access

IEEE 802.11 MAC Timing Basic Access Method
Stallings DCC8e Figure 17.7a illustrates the use of these time values.

SIFS Use SIFS gives highest priority
over stations waiting PIFS or DIFS time SIFS used in following circumstances: Acknowledgment (ACK) station responds with ACK after waiting SIFS gap for efficient collision recovery (there is no collision detection) and multi-frame transmission Clear to Send (CTS) station ensures data frame gets through by issuing RTS and waits for CTS response from destination Poll response see Point Coordination Function (PCF) discussion later Consider first the SIFS. Any station using SIFS to determine transmission opportunity has, in effect, the highest priority, because it will always gain access in preference to a station waiting an amount of time equal to PIFS or DIFS. The SIFS is used in the following circumstances: • Acknowledgment (ACK): When a station receives a frame addressed only to itself (not multicast or broadcast), it responds with an ACK frame after waiting only for an SIFS gap. This has two desirable effects. First, because collision detection is not used, the likelihood of collisions is greater than with CSMA/CD, and the MAC-level ACK provides for efficient collision recovery. Second, the SIFS can be used to provide efficient delivery of an LLC protocol data unit (PDU) that requires multiple MAC frames. In this case, the following scenario occurs. A station with a multiframe LLC PDU to transmit sends out the MAC frames one at a time. Each frame is acknowledged by the recipient after SIFS. When the source receives an ACK, it immediately (after SIFS) sends the next frame in the sequence. The result is that once a station has contended for the channel, it will maintain control of the channel until it has sent all of the fragments of an LLC PDU. • Clear to Send (CTS): A station can ensure that its data frame will get through by first issuing a small Request to Send (RTS) frame. The station to which this frame is addressed should immediately respond with a CTS frame if it is ready to receive. All other stations receive the RTS and defer using the medium. • Poll response: This is explained in the following discussion of PCF.

PIFS and DIFS Use PIFS used by centralized controller
for issuing polls has precedence over normal contention traffic but not SIFS DIFS used for all ordinary asynchronous traffic The next longest IFS interval is the PIFS. This is used by the centralized controller in issuing polls and takes precedence over normal contention traffic. However, those frames transmitted using SIFS have precedence over a PCF poll. Finally, the DIFS interval is used for all ordinary asynchronous traffic.

IEEE MAC Frame Format Stallings DCC8e Figure 17.8 shows the frame format. This general format is used for all data and control frames, but not all fields are used in all contexts. The fields are: • Frame Control: Indicates the type of frame (control, management, or data) and provides control information. Control information includes whether the frame is to or from a DS, fragmentation information, and privacy information. • Duration/Connection ID: If used as a duration field, indicates the time (in microseconds) the channel will be allocated for successful transmission of a MAC frame. In some control frames, this field contains an association, or connection, identifier. • Addresses: The number and meaning of the 48-bit address fields depend on context. The transmitter address and receiver address are the MAC addresses of stations joined to the BSS that are transmitting and receiving frames over the wireless LAN. The service set ID (SSID) identifies the wireless LAN over which a frame is transmitted. • Sequence Control: Contains a 4-bit fragment number subfield, used for fragmentation and reassembly, and a 12-bit sequence number used to number frames sent between a given transmitter and receiver. • Frame Body: Contains an MSDU or a fragment of an MSDU. The MSDU is a LLC protocol data unit or MAC control information. • Frame Check Sequence: A 32-bit cyclic redundancy check.

Point Coordination Function (PCF)

PCF Superframe Timing Stallings DCC9e Figure 17.7b illustrates the use of the superframe. At the beginning of a superframe, the point coordinator may optionally seize control and issue polls for a given period of time. This interval varies because of the variable frame size issued by responding stations. The remainder of the superframe is available for contention-based access. At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS. If the medium is idle, the point coordinator gains immediate access and a full superframe period follows. However, the medium may be busy at the end of a superframe. In this case, the point coordinator must wait until the medium is idle to gain access; this results in a foreshortened superframe period for the next cycle.

Point Coordination Function (PCF)
alternative access method implemented on top of DCF polling by centralized polling master (point coordinator) uses PIFS when issuing polls point coordinator polls in round-robin to stations configured for polling when poll issued, polled station may respond using SIFS if point coordinator receives response, it issues another poll using PIFS if no response during expected turnaround time, coordinator issues poll coordinator could lock out asynchronous traffic by issuing polls have a superframe interval defined PCF is an alternative access method implemented on top of the DCF. The operation consists of polling by the centralized polling master (point coordinator). The point coordinator makes use of PIFS when issuing polls. Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses. As an extreme, consider the following possible scenario. A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic contends for access using CSMA. The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. When a poll is issued, the polled station may respond using SIFS. If the point coordinator receives a response, it issues another poll using PIFS. If no response is received during the expected turnaround time, the coordinator issues a poll. If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls. To prevent this, an interval known as the superframe is defined. During the first part of this interval, the point coordinator issues polls in a round-robin fashion to all stations configured for polling. The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access.

PCF Examples 46

Control Frames 6 control frame types:
Power Save-Poll (PS-Poll): sent by any station to the AP; request that the AP transmit a frame that has been buffered for this station while the station was in power-saving mode. RTS CTS ACK Contention-free end (CF-End): sent by the PC to announce the end of a contention-free period. CF-End + CF-ACK: sent by the PC; combined a CF-End with ACK of the data frame the PC last received. We now look at the three MAC frame types. CONTROL FRAMES Control frames assist in the reliable delivery of data frames. There are six control frame subtypes: Power Save-Poll (PS-Poll): This frame is sent by any station to the station that includes the AP (access point). Its purpose is to request that the AP transmit a frame that has been buffered for this station while the station was in power-saving mode. Request to Send (RTS): This is the first frame in the four-way frame exchange discussed under the subsection on reliable data delivery at the beginning of Stallings DCC9e Section The station sending this message is alerting a potential destination, and all other stations within reception range, that it intends to send a data frame to that destination. Clear to Send (CTS): This is the second frame in the four-way exchange. It is sent by the destination station to the source station to grant permission to send a data frame. Acknowledgment: Provides an acknowledgment from the destination to the source that the immediately preceding data, management, or PS-Poll frame was received correctly. Contention-Free (CF)-end: Announces the end of a contention-free period that is part of the point coordination function. CF-End + CF-Ack: Acknowledges the CF-end. This frame ends the contention-free period and releases stations from the restrictions associated with that period.

Data Frames – Data Carrying
8 data frame types organized in two groups first four carry upper-level data remaining do not carry any user data Data simplest data frame, contention or contention-free use Data + CF-Ack carries data and acknowledges previously received data during contention-free period Data + CF-Poll used by point coordinator to deliver data and poll (request send) Data + CF-Ack + CF-Poll combines Data + CF-Ack and Data + CF-Poll There are eight data frame subtypes, organized into two groups. The first four subtypes define frames that carry upper-level data from the source station to the destination station. The four data-carrying frames are: Data: This is the simplest data frame. It may be used in both a contention period and a contention-free period. Data + CF-Ack: May only be sent during a contention-free period. In addition to carrying data, this frame acknowledges previously received data. Data + CF-Poll: Used by a point coordinator to deliver data to a mobile station and also to request that the mobile station send a data frame that it may have buffered. Data + CF-Ack + CF-Poll: Combines the functions of the Data + CF-Ack and Data + CF-Poll into a single frame.

Data Frames – Not Data Carrying
Null function carries no data, polls, or acknowledgments carries power management bit in frame control field sent to AP indicates station is changing to low-power state Other three frames CF-Ack, CF-Poll, CF-Ack + CF-Poll same as corresponding frame in the preceding list but without data. The remaining four subtypes of data frames do not in fact carry any user data. The Null Function data frame carries no data, polls, or acknowledgments. It is used only to carry the power management bit in the frame control field to the AP, to indicate that the station is changing to a low-power operating state. The remaining three frames (CF-Ack, CF-Poll, CF-Ack + CF-Poll) have the same functionality as the corresponding data frame subtypes in the preceding list (Data + CF-Ack, Data + CF-Poll, Data + CF-Ack + CF-Poll) but without the data.

PCF Examples

used to manage communications between stations and APs
Management Frames used to manage communications between stations and APs management of associations requests, response, reassociation, dissociation, and authentication Management frames are used to manage communications between stations and APs. Functions covered include management of associations (request, response, reassociation, dissociation, and authentication.

IEEE Physical Layer

Physical Layer The physical layer for IEEE has been issued in five stages. The first part, simply called IEEE , includes the MAC layer and three physical layer specifications, two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1 and 2 Mbps. IEEE a operates in the 5-GHz band at data rates up to 54 Mbps. IEEE b operates in the 2.4-GHz band at 5.5 and 11 Mbps. IEEE g also operates in the 2.4-GHz band, at data rates up to 54 Mbps. Finally, IEEE n operates in either the 2.4-GHz band or the 5-GHz band with data rates in the hundreds of Gbps. Stallings DCC9e Table 17.4 provides some details. We look at each of these in turn.

Data Rate vs. Distance (m)
The IEEE standards do not include a specification of speed versus distance objectives. Different vendors will give different values, depending on environment. Stallings DCC9e Table 17.5, based on [LAYL04] gives estimated values for a typical office environment.

802.11n IEEE 802.11n has enhancements in 3 general areas:
multiple-input-multiple-output (MIMO) antenna architecture most important enhancement radio transmission scheme increased capacity MAC enhancements most significant change is to aggregate multiple MAC frames into a single block for transmission IEEE n With increasing demands being placed on wireless LANs, the committee looked for ways to increase the data throughput and overall capacity of networks. The goal of this effort is to not just increase the bit rate of the transmitting antennas but to increase the effective throughput of the network. Increasing effective throughput involves not only looking at the signal encoding scheme, but also at the antenna architecture and the MAC frame structure. The result of these efforts is a package of improvements and enhancements embodied in IEEE n. This standard is defined to operate in both the 2.4-GHz and the 5-GHz bands and can therefore be made upwardly compatible with either a or b/g. IEEE n embodies changes in three general areas: use of MIMO, enhancements in radio transmission, and MAC enhancements. We briefly examine each of these. Multiple-input-multiple-output (MIMO) antenna architecture is the most important of the enhancements provided by n. A discussion of MIMO is beyond our scope, so we content ourselves with a brief overview (see Stallings DCC9e Figure 17.11). In a MIMO scheme, the transmitter employs multiple antennas. The source data stream is divided into n substreams, one for each of the n transmitting antennas. The individual substreams are the input to the transmitting antennas (multiple input). At the receiving end, m antennas receive the transmissions from the n source antennas via a combination of line-of-sight transmission and multipath. The outputs from the m receiving antennas (multiple output) are combined with the signals from the other receive radios. With a lot of complex math, the result is a much better receive signal than can be achieved with either a single antenna or multiple frequency channels n defines a number of different combinations for the number of transmitters and the number of receivers, from 2  1 to 4  4. Each additional transmitter or receiver in the system increases the SNR (signal-to-noise ratio). However, the incremental gains from each additional transmitter or receiver diminish rapidly. The gain in SNR is large for each step from 21 to 22 and to 32, but the improvement with 33 and beyond is relatively small [CISC07]. In addition to MIMO, n makes a number of changes in the radio transmission scheme to increase capacity. The most significant of these techniques, known as channel bonding, combines two 20-MHz channels to create a 40-MHz channel. Using OFDM, this allows for twice as many subchannels, doubling the transmission rate. Finally, provides some MAC enhancements. The most significant change is to aggregate multiple MAC frames into a single block for transmission. Once a station acquires the medium for transmission, it can transmit long packets without significant delays between transmissions. The receiver sends a single block acknowledgement. Frame aggregation can result in significantly improved efficiency in the use of the transmission capacity. Figure gives an indication of the effectiveness of n compared to g [DEBE07]. The chart shows the average throughput per user on a shared system. As expected, the more active users competing for the wireless capacity, the smaller the average throughput per user. IEEE n provides a significant improvement, especially for networks in which a small number of users are actively competing for transmission time.

Summary IEEE 802.11 architecture 802.11 Medium Access Control
Physical Layers 802.11, a, b, g, n Stallings DCC9e Chapter17 summary.

Reference Chapter 17, Stallings

Wireless LANs and IEEE MAC

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